Fix Markdown mark-up and add WebUI info

Rendering this table in Gitiles requires a table header.

I've checked rendering using "gitiles" link in:
https://chromium-review.googlesource.com/c/chromium/src/+/1223347/9/docs/test_descriptions.md
https://chromium.googlesource.com/chromium/src/+/refs/changes/47/1223347/5/docs/test_descriptions.md

Add more information for WebUI tests.

Bug: 880688
Change-Id: If19bada962b67899676ebd2e4d8fa19b9fcec381
Reviewed-on: https://chromium-review.googlesource.com/1223347Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592647}

docs/test_descriptions.md

 See [Testing and infrastructure](https://sites.google.com/a/chromium.org/dev/developers/testing) for more information.
 
-|:-----------------------|:-|
+| Type of test           | Description |
+|:-----------------------|:------------|
 |accessibility\_unittests| |
 |angle\_unittests        | |
 |app\_list\_unittests    | |
@@ -56,3 +57,4 @@ See [Testing and infrastructure](https://sites.google.com/a/chromium.org/dev/dev
 |views\_unittests        | |
 |wav\_ola\_test          | |
 |webkit\_unit\_tests     | |
+|webui tests             | Special type of browser\_tests used for [WebUI features](https://chromium.googlesource.com/chromium/src/+/master/docs/webui_explainer.md), see [here](https://docs.google.com/document/d/1Z18WTNv28z5FW3smNEm_GtsfVD2IL-CmmAikwjw3ryo/edit#) for more information on known issues with WebUI test infrastructure. |